The Justice Department was running into immediate hurdles Monday in its investigation of possible civil rights violations by George Zimmerman in the shooting death of Trayvon Martin -- namely, that after examining the case for more than a year, the evidence has not changed.

Though the department announced after Zimmerman's acquittal that it would consider a possible federal case, previously filed FBI documents show agents have not turned up any accounts that Zimmerman, before the February 2012 shooting, exhibited racial bias. Sanford police detective Chris Serino also told FBI agents last year that he considered Zimmerman as having "a little hero complex, but not as a racist."

Attorney General Eric Holder, in his first post-verdict comments, confirmed Monday during a speech in Washington, D.C., that his department continues to investigate while signaling concern for the position of the Martin family and those -- such as the NAACP -- pressuring the DOJ. He said the department is "mindful of the pain felt by our nation" over the "tragic, unnecessary shooting death" of Martin.

He added that the shooting provides an opportunity to speak "honestly" about the charged issues involved in the case, and that "we must not ... let this opportunity pass." Holder even appeared to suggest the possibility of bias in this case, saying it's important to address "underlying attitudes, mistaken beliefs and stereotypes that serve as the basis for these too common incidents."

Still, the attorney general said his department would act in a "manner that is consistent with the facts and the law."

White House Press Secretary Jay Carney indicated that President Obama would keep his distance from the consideration, adding that the merits of the case will be "evaluated by the professionals at the Department of Justice."

But Florida defense attorney Brian Tannebaum told FoxNews.com that the "number one challenge" for DOJ is the evidence, or lack thereof.

"There's been an acquittal. The evidence has not changed. It's not like the feds are going to go in and find more evidence," he said.

Tannebaum said he understands the Justice Department was responding to intense public sentiment, but "I don't foresee it becoming a federal case."

The question of racial bias is key because if the Justice Department were to pursue a federal case against Zimmerman, federal hate-crimes law is one of the few tools the department would have.

The statute itself bars "willfully" causing injury to someone else "because of the actual or perceived race, color, religion, or national origin of any person."

The department has used federal civil rights law before to pursue defendants who had been acquitted at the state level. But in what is perhaps the most famous such case -- the Rodney King beating -- the circumstances were different.

First, there was a highly publicized video of the King beating. No such video of the Martin shooting is known to exist.

Second, the two officers ultimately convicted on federal charges in that case were charged with using or permitting unreasonable force under "color of law" -- this applies only to law enforcement.

The separate federal hate-crimes law applies to both law enforcement and civilians. But it's not clear how the Justice Department could prove racial bias.

Serino, the Sanford detective, told FBI agents last year that after examining the case, it appeared Zimmerman was suspicious of Martin because of his "attire, the total circumstances of the encounter and the previous burglary suspects in the community."

Serino told the FBI that there had been several burglaries in the area, and that gang members in the community "typically dressed in black and wore hoodies."

"When Zimmerman saw Martin in a hoody, Zimmerman took it upon himself to view Martin as acting suspicious," Serino said, while describing Zimmerman as "overzealous." The FBI document was posted on the Smoking Gun website.

McClatchy also has reported on another set of documents that show FBI agents interviewed a dozens of people in the course of probing possible racial bias but nobody would say Zimmerman showed such bias before the shooting.

Still, the Justice Department agreed to requests from NAACP President Benjamin Todd Jealous and several lawmakers to keep investigating the defendant.

"The Department of Justice's Criminal Section of the Civil Rights Division, the United States Attorney's Office for the Middle District of Florida, and the Federal Bureau of Investigation continue to evaluate the evidence generated during the federal investigation, as well as the evidence and testimony from the state trial," the Justice Department said in a statement Sunday. "Experienced federal prosecutors will [now] determine whether the evidence reveals a prosecutable violation of any of the limited federal criminal civil rights statutes within our jurisdiction, and whether federal prosecution is appropriate in accordance with the Department's policy governing successive federal prosecution following a state trial."

The department may find itself in a vulnerable spot, not only because of prior interviews which found no evidence of bias.

Last week, a conservative watchdog accused an obscure agency within the DOJ of helping support the "pressure campaign" against Zimmerman in the wake of the shooting last year. Judicial Watch claimed documents and public accounts showed "extraordinary intervention" by the department in the campaign that eventually led to Zimmerman's prosecution.

The department, however, claims that it dispatched agency representatives to reduce tensions in the community - not to take sides.

The department is under heavy pressure from some groups to pursue the case further.

Rep. Charles Rangel, D-N.Y., and other House Democrats called a press conference on Monday to urge the Justice Department to "conduct an immediate and thorough investigation."

The NAACP started the drive to pressure the DOJ to continue its probe by posting a petition Sunday morning on the website MoveOn.org.

"The most fundamental of civil rights -- the right to life -- was violated the night George Zimmerman stalked and then took the life of Trayvon Martin," the group wrote.

Attorney General Eric Holder plans to speak at the NAACP convention on Tuesday.